1. A reference to the Woodstock Festival in 1969, when stage announcements were made by legendary radio disc jockey Wavy Gravy, telling people that some of the brown-colored blotter acid (LSD) circulating the crowd was giving people negative hallucinations, causing freakouts.
2. A bad drug experience
3. Something that was supposed to be pleasurable but turned out nightmarish.

1. I thought this job was going to be fun, but it turned out to be, like, "Dude, don't take the brown acid."
2. That chick is the brown acid, but hey, it's up to you.

Most notably made widely known to the masses with the famous quote at Woodstock in 1969.

It wasn't certain that the brown acid the was circulating there was "bad", but because of the number of people that were "tripping" - many for the first time - there were many that were experiencing a "bad trip".

"To get back to the warning that I received. You may take it with however many grains of salt that you wish. That the brown acid that is circulating around us isn't too good. It is suggested that you stay away from that. Of course it's your own trip. So be my guest, but please be advised that there is a warning on that one, ok?"

Not spoken by Wavy Gravy, but perhaps by Chip Monk. More research is needed to confirm this fact.